This invention relates generally to novel data gathering and sampling in connection with soil mechanics. More particularly, this invention concerns a method and apparatus for sampling and determining the dynamic loading characteristics of a soil bed, and more particularly, a method for measuring, as a function of time, the force and displacement on a soil sample as the apparatus presses into a soil bed at an uncontrolled rate resulting in a variable penetration rate. The apparatus may be used in connection with a sub-sea soil bed or a soil bed on land.
In the past it has been common practice to extract soil samples and make laboratory measurements of data concerning the characteristics of a soil bed on the recovered samples. While some arrangements have exhibited at least a degree of utility in the gathering of data in connection with soil mechanics analysis, room for significant improvement remains.
The structural loading of soil has been a problem for many years, but these problems were not approached in an orderly manner until the advent of modern soil mechanics theory in the 1920's. The application of soil mechanics theory requires the collection of accurate data to evaluate certain soil parameters. The task of gathering reliable data is of paramount importance in the satisfactory application of soil mechanics theory. This task becomes acutely more difficult when analyzing a soil bed that lies beneath a body of water.
As the world's oil supply dwindles and available land based drilling sites are exhausted, the need to construct offshore oil drilling platforms increases. The increased size and utilization of these offshore platforms magnifies the need for reliable data to evaluate the stability of sub-sea soil beds. Offshore platforms constructed on pilings driven into the soil bed under bodies of water proliferate in the Gulf of Mexico and along the continental shelf bordering the east and west coasts of the United States.
Data taken while sampling a soil bed helps determine the soil bed's ability to support the foundation of a structure. A foundation is only as stable as the soil bed that supports it. Accurate data collection concerning a soil bed is the first step in correctly evaluating the soil bed's ability to support a structural foundation. A stable foundation is fundamental to the stability of a structure. The need for accurate design data is paramount. A calculation based on erroneous data is a miscalculation that can produce disastrous results. A structure built upon a piling foundation, subjected to a sudden load from a wave surge or earthquake, can collapse, resulting in a loss of life and property.
The ability of a soil bed to support a structure's foundation is related to the rate a load is applied to the foundation. While a soil bed may adequately support a foundation during normal wave activity, or normal land based loading, the soil bed may not adequately support the foundation during a sudden surge in response to severe wave action or an earthquake. An unexpected load applied suddenly to the foundation could topple the structure. Therefore, there is an important need to accurately predict the ability of a soil bed to support a structure, especially during the variable rate loading conditions experienced on land and at sea. Variable rate loading characteristics are referred to as the dynamic loading characteristics of the soil bed.
Present methods and apparatus for measuring the ability of a soil bed to support a structure are limited in several ways. First, there are no known methods or apparatus that measure the dynamic loading characteristics of a soil bed as a function of time. Moreover, present methods and apparatus utilize short displacement, cyclic, linear penetration techniques that penetrate a soil bed at a constant rate and do not measure the dynamic loading characteristics of the soil.
Known measuring systems are intolerant of a hostile sea state and require a benign sea state to obtain accurate data. Unless these methods and apparatus are used in smooth water conditions, motion compensation devices must be used to obtain accurate measurements.
Physical interface umbilicals from the surface are difficult to deploy and present a formidable, if not impossible, design challenge in deep sea applications. In addition the tremendous pressure exerted on equipment and instrumentation submerged in over five hundred fathoms of water presents a formidable design problem.
Isolating a monitoring system from extreme water pressure and from the corrosive action of the sub-sea environment is extremely difficult. These problems are exacerbated by the use of physical umbilicals.
The problems enumerated in the forgoing are not exhaustive but rather are among many which tend to impair the effectiveness of previously known soil sampling and data gathering systems. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that soil sampling and data gathering systems appearing in the art have not been altogether satisfactory.